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Untangling the plasma filaments mystery…with a trip to the movies | 07/11/2016

These are some of the most detailed images ever of the intensely hot plasma inside a tokamak. They're helping scientists understand how to contain fusion reactions in the power stations of the future – and, importantly, how to protect their walls from damage.

At 100,000 frames per second, the movies from Culham's MAST device give a vivid illustration of how tokamaks keep fusion fuel trapped in a magnetic ‘cage'. The particles moving around the field lines resemble a large ball of wool being spun together. If only it were that simple; in reality, a magnetically-confined plasma is a highly complex system, and predicting how it behaves is key to making nuclear fusion a viable energy source. In particular, knowing how the hot fuel affects the cold walls of the machine is integral to ensuring that future reactors survive.

Turbulence in the magnetic field throws out wispy bunches of particles – known as filaments – from the plasma in a seemingly random fashion, ejecting fuel which touches the surfaces of the tokamak. Researchers are now working to unravel meaning within this randomness to understand this complex interaction with the machine walls, and videos such as these can give them pointers to what is happening.

Nick Walkden of CCFE's Theory & Modelling Department, who produced the videos, explains: “We believe that filaments are a vital part of the ‘exhaust process' within a tokamak – how particles are expelled from the plasma. Seeing the MAST plasma at this unprecedented level of detail enables us to image individual filaments and measure their size, velocity and position within the plasma. It tells us a lot about their physics so we can find out how to predict their motion and, in future experiments, possibly learn to control them.”

Nick combines the images with computer overlays of the inside of the MAST tokamak so researchers know where the plasma is in relation to the inner wall. This is particularly important because filaments can sometimes damage plasma-facing machine components.

He has also created a ‘synthetic camera' which replicates the view of the real camera to visualise fusion physicists' theoretical predictions of filament formation and movement. Comparing the videos from the real and ‘synthetic' cameras allows them to test their simulations against the real experimental data in a like-for-like way.

“This essentially gives us data for free!” says Nick. “It means we can marry up what we think should be happening, based on our theoretical models, with what is actually happening in the plasma. As we refine our theories, we can run new simulations in the synthetic camera and see how closely they map with the real videos. Our EUROfusion research partners have supplied simulations using the most advanced codes from around Europe to run with the synthetic camera, which has given us encouraging results. We have high resolution videos from about 400 MAST plasma experiments, so there are plenty of ‘real' filaments to measure against.

“It's the first time this kind of validation has been done in a tokamak – we hope it could really take this area of study forward.”

Images

Top: High resolution video of MAST plasma pulse 29841, showing filaments moving along magnetic field lines at the edge of the plasma

Above: (1) Still image from the same pulse video with artificial colouring. A computer overlay of MAST’s inner wall indicates the plasma’s positioning within the tokamak; (2) Screen shot from a synthetic camera's ‘theoretical’ video of a MAST plasma, showing 42 randomly distributed filaments of differing shapes and sizes